In applications where apparatuses and mechanical components are subjected to extreme conditions such as high steam pressure and high thermal stresses, wear and galling of the article can adversely affect the reliability and life of the apparatus components. Steam turbine valves are just one example of a component which has an increased likelihood of galling or significant wear on its sliding and contact surfaces due to extreme steam turbine operating conditions. Valve components have frequent opening and closing cycles, high impact loads, and significant steam pressure during cold, warm or hot start of a steam turbine. These repeated stresses exacerbate the wear and galling of steam turbine valves. The susceptible surfaces include, but are not limited to, valve disks, valve seats, valve stems, valve bushings, valve disks, and balance chambers.
The results of galling and wear of steam turbine valves can include leaks, the inability to open the valve causing failure to produce and deliver power to a grid, an inability to isolate the steam path, and a severely compromised seal of the steam path in the valve which could cause a turbine overspeed event during emergency shutdown conditions. Other examples of articles which may be susceptible to wear and galling include, but are not limited to, airfoil buckets, nozzles, and turbines.
Previously, nitrides or thermally sprayed carbides have been applied to articles to prevent wear and galling. However, nitrided surface/layer coatings can only be applied to certain classes of materials and nickel-based alloys cannot be gas, plasma or bath nitrided by traditional means. In addition, these nitride coatings provide a hardness of about 1,000 Vickers, which does not result in adequate wear and galling resistance. Furthermore, forming nitrides with unsuitable (i.e., un-nitridable) alloys can severely reduce the corrosion resistance of some of the alloys. Nitriding can also be reversible above about 1100° F., resulting in loss of surface hardness. Accordingly, there is a need for a simple and economically desirable wear and galling resistant article for use in various conditions such as high pressure and high thermal stress conditions.